Troubleshooting Linux

Technical support analysts often receive tickets about high server loads. The cause of high server loads is very rarely attributed to defects in the cPanel software or the applications it installs. High server loads are something that should be initially investigated by the server owner, their system administrator, or server provider.

What causes high server loads?

Excessive usage of any of the following items can typically cause this issue:

CPU

memory (including swap)

disk I/O

How can I check these items?

That depends whether you want to review their current resource usage, or historical resource usage. This tutorial will cover both.

A brief lesson on “sar”

Historical resource usage can be viewed using the “sar” utility, which should exist by default on all cPanel servers from the sysstat package. The stats are collected when sysstat runs from cron (/etc/cron.d/sysstat). If crond is not running, sysstat will not be able to collect historical statistics.

To view resource usage histories from sar, you must provide the path to the file that corresponds with the date of the stats.

For example, if you wanted to view the load averages for your server from the 23rd of the month, you would run this command:Code:

[user@host ~]$ sar -q -f /var/log/sa/sa23

The command above uses ‘-q’ to obtain the load average information, and ‘-f’ to specify which sar file to obtain the information from. Note that sar may not have historical data going back more than a week or so.

You do not need to specify the date when viewing the statistics for the current day. As such, this command would show the load average for today:Code:

[user@host ~]$ sar -q

You are strongly encouraged to read the documentation for sar:Code:

[user@host ~]$ man sar

It provides statistics for many things that can be helpful to know about.

Current CPU usage

Run “top”, and on the line that says “Cpu(s)”, check the “%id” section which shows the percentage of which your CPUs are idle. The higher the number the better. A 99% idle CPU is not doing much of anything, and a 1% idle CPU is heavily tasked.Code:

[user@host ~]$ top c

Tip: hit “P” to sort by processes that are currently consuming the most CPU.

Historical CPU usage

Check the “%idle” column:Code:

[user@host ~]$ sar -p

Current memory usageCode:

[user@host ~]$ free -m

Tip: run “top c” and hit “M” to see which processes are consuming the most memory.

Historical memory usage

This depends on the version of sar, which used to use ‘-r’ to show %memused and %swpused (swap memory used), but later changed to ‘-S’ to show %swpused.

Check “%memused” and “%swpused”:Code:

[user@host ~]$ sar -r

OR:Code:

[user@host ~]$ sar -r

Code:

[user@host ~]$ sar -S

A note about memory usage: it is normal to see much of the server’s memory being used. Why? Because the OS loves to cache things in memory. Why? Because accessing data from memory is extremely fast and far more efficient than using the server’s disk(s).

As such, %memused isn’t generally going to be much of an issue (unless perhaps you don’t have a swap partition, but that’s an issue in and of itself). You should focus on %swpused, which is what gets used when your server’s physical memory is full. The lower the number, the better. A %swpused percentage of 0% would mean that your server currently has sufficient physical memory to perform its tasks.

How much %swpused is too much? That depends on your opinion of “too much”. Generally speaking, a consistent low percentage of swap usage may not be an issue on your server. If you observe the %swpused increasing over time (e.g., from 1%, to 7%, to 32%), something on your server is consuming too much memory, and it would be wise to determine what that is (rather than just installing more memory). If your server ends up using all of its physical memory and swap memory, it may become unresponsive, requiring a reboot.

Current disk I/O usage

Note: this does not work on OpenVZ/Virtuozzo containers.

This will print the disk usage statistics 10 times, every 1 seconds. Check the %util column:Code:

[user@host ~]$ iostat -x 1 10

Historial disk I/O usageCode:

[user@host ~]$ sar -d

Good system administration involves knowing when your server’s load is higher than acceptable. The main reason for this (other than preventing your server from becoming unresponsive and requiring a reboot) is to see what’s taking place on the server while the load is high. Fast actions will enable you to troubleshoot the issue while it is occurring.

If your server’s load was high from 2AM – 4AM while you were sleeping, you would have missed what took place. While sar can be helpful to show you what specific resources were high during that time, it won’t tell you the cause of the high usage. There can be many causes, including DoS attacks, spam attacks, poorly designed php scripts which consume large amounts of memory, web spiders that crawl sites too aggressively, hardware issues, massive amounts of disk writes to a user’s MySQL database, and much, much more.

The good news is that you can have much of this information collected and sent to you automatically while the load is high, which you can review later as needed. How? From your process list:Code:

[user@host ~]$ ps auxwwwf

I have created a shell script for this, which is based off of a perl script that I used to run on servers that I managed. It was very useful to me in conjunction with other server monitoring (such as via Nagios). It checks 6 different things (more on this below), and emails you the current process list if any of them exceed your specific threshold.

This script is not developed, maintained, or supported by cPanel, Inc. Please do not open tickets about this script. If you experience any issues using it and require assistance, you can post a reply here, or consult an experienced system administrator. cPanel cannot provide support for this script.

The resources that are checked are as follows:

1 minute load average

kilobytes of swap used

kilobytes of memory usage

packets per second inbound

packets per second outbound

number of processes

How to use the script

To run the script automatically, set up a cron job that executes it as often as you’d like. I found every 5 minutes to be a good fit. The script does not need to be run as root, so do not run it as root.

If one of the resources has exceeded its user defined threshold, the script will send you an email that contains the current process list (ps auxwwwf).

IMPORTANT: You will need to adjust the values to your liking. There are no perfect default values. Why? Because different server environments are, well, different. For example, it may be preferred to set the 1 minute load average threshold higher for a server with 16 CPU cores than a server with just 1.

NOTE: You will need to add your email address to the “EMAIL” variable. For example:Code:

Please feel free to post questions, comments, and anything else about troubleshooting server loads by clicking on the Discussion tab. This resource will inevitably be missing some other useful troubleshooting items, and your comments are encouraged.